rebase of PR #1426: Fixes Issue 8765 & 9255.

https://github.com/D-Programming-Language/dmd/pull/1426
http://d.puremagic.com/issues/show_bug.cgi?id=8765
http://d.puremagic.com/issues/show_bug.cgi?id=9255
Issue 8765 - Assert should print failing expression.
Issue 9255 - Assert should print file name rather than module name